More generator questions

Ok, thought my generator was shot.   took it out today after reading a few articles and learning some more on diagnosing it.   I read this article on YT forum.

Powertech is right about polarity. A generator doesn't care about polarity. Polarizing it is kind of like telling it which pole is ground. If you can make it motor, it's ok. The regulator usually does just 2 things. It has the cutout relay and the voltage regulator in it. Some of the larger capacity generators in automotive applications also had a current limiting function.
The YT article sends you out around Robin Hood's barn in testing a generator. You want to do the motor test by first running battery voltage in the "A" terminal and then ground the "F" terminal. If you run power in the A and ground directly through the F, you could have a shorted field and not know it because you wouldn't see the generator slow down when the field is grounded.
Without a "growler", you can't test the armature (part that spins) with the generator disassembled. An internally shorted armature shows up on the growler by holding a hacksaw blade over it and the blade will vibrate.
The regulator is fairly simple. The regulator has a coil of very fine wire wrapped around an iron core making an electromagnet. There are a set of points that are held in the closed position by a spring. The points make and break the ground path for the field circuit. As the voltage rises, the electromagnet gets stronger and opens the points. When the magnet gets weaker, the points close and the cycle repeats many times per second. In the automotive world we call that "duty cycle". The higher the duty cycle, the longer the circuit is closed. 100% would be on all the time. 50% would be on half the time and off the other half.
The cutout relay is simply an on off switch. If there were no cutout relay, when the engine stops, the battery would discharge through the generator. Remember the motoring test? The generator always has some residual magnetism and will generate a small amount of power. This is enough to energize an electromagnet under the cutout relay points and pull them closed so the generator can charge the battery.


Edited by Brian Jasper co. Ia - 19 Mar 2010 at 10:57pm

take a hack saw blade and clean the mica out too make it a little deepre too

Showing no conductivity from A terminal is NOT GOOD. That should show continuity through the armature and brushes. My guess is that the brushes are worn to the point of not making contact or a flex wire on a brush is broken. There should be continuity from F to frame also, though it should be an ohm or two greater than the continuity from A to frame which should be a fraction of an ohm.

Gerald J.

Good eye Gerald, I missed that. No continuity between the A terminal and the frame is a problem.

Rememmber to polorize after installation. It may take more then one flash to get it polorized and to work. You can also flash it while the tractor is running. Bob

Sorry Chuck, coming into this way late, but from reading your posts, sounds to me like a possible bad arm is the culprit here....
Steve@B&B

I was wondering what chalmersbob is refering to when he is saying to polorize and flash the generator when the tractor is running.